The 33 trapped Chilean miners who have astonished the world with their discipline a half mile underground will have to aid their own escape by clearing the tons of rock that will fall as the rescue hole is drilled, the engineer in charge of drilling has said.
After drilling three small bore holes in recent weeks to create lines of communication with the miners and deliver basic food and medicine, Chile's state-owned Codelco mining company will begin boring a rescue hole on Monday afternoon that will be wide enough to pull the men up through 700 metres of earth.
The first step will be to drill a "pilot hole" similar in size to the other three. Then much larger machine cutters will slowly grind through that hole, forcing thousands of tons of crushed rock to fall down into the mine shaft area near the trapped men.
Failure to keep the bottom clear of debris could quickly plug the hole, delaying a rescue that officials say could take three to four months.
"The miners are going to have to take out all that material as it falls," Andres Sougarret, Codelco's head engineer on the operation, told The Associated Press in a phone interview.
In all, the trapped miners will have to clear between 3000 and 4000 tonnes of rock, work that will require crews of about a half-dozen men working in shifts 24 hours a day.
The men have basic clearing equipment, such as wheelbarrows and industrial-sized battery-powered sweepers, Sougarret said. The hole will likely end up several hundred metres from their living area in the mine's shelter, giving the men room to maneuver and store the rocks, he added.
Sougarret declined to estimate how long the work would take, saying it would depend on how each step went.
Once drilling begins, the team will have to decide whether to fit the wider hole with metal casing, often used to seal a hole and prevent collapses in the walls.
"We may not have to use it in this case because the rock is really high quality, really strong," he said.
On Sunday, Mining Minister Laurence Golborne reiterated the government's estimate of three to four months to rescue the men, rejecting local reports citing engineers who said it could be done in much less time.
Golborne said that experts had analysed 10 different methods to get the men out, will continue to study other options, but that "nothing has yet been found that will be quicker".
It's unclear if the government is simply trying to under-promise and then over-deliver, but there is widespread agreement that the major drilling operation is unlikely to endanger the miners.
"If the area where the miners are didn't get crushed in the initial collapse, drilling this new hole isn't going to do that," Walter Veliz Araya, the geologist who was in charge of drilling the three bore holes, said.
Mario Medina Mejia, a Chilean mining engineer not involved in the operation, agreed.
"The question isn't whether they can safely get to the miners," Mejia said. "It's how long can the miners wait for them to arrive?"
Normally, after completing a pilot hole, the opening is enlarged by drilling from the bottom up. The drill, hanging at the bottom of the pilot hole, is reached through existing shafts in a mine and then fitted with the machine cutters, which then blast through rock as they are raised.
In this case, however, there is no way to get those large cutters to the bottom of the mine; if there was a hole large enough to reach it, the men would already have been rescued.
Araya said that knowledge gained drilling the initial holes, which are between 20 and 100 metres from the shelter, would give the team digging the rescue hole a head start. For example, while penetrating rock, the circular motion of the bits causes the drill to veer right. In this case, the especially hard rock exaggerates that, making constant correction necessary, he said.
From the moment the mine collapsed on August 5, the trapped men have had a central role in keeping themselves alive - getting to the safety chamber, rationing food and keeping order with extraordinary discipline.
In video footage released by the government late on Thursday, one of the miners says proudly that they will be helping with the operation, a sign that authorities have already prepared them.
